/*
 * Copyright (C) 2009 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.jn.langx.escape;

import com.jn.langx.annotation.Nullable;
import com.jn.langx.util.Preconditions;

import java.util.HashMap;
import java.util.Map;

/**
 * Static utility methods pertaining to {@link Escaper} instances.
 */
public final class Escapers {
    private Escapers() {
    }

    /**
     * Returns an {@link Escaper} that does no escaping, passing all character data through unchanged.
     */
    public static Escaper nullEscaper() {
        return NULL_ESCAPER;
    }

    // An Escaper that efficiently performs no escaping.
    // Extending CharEscaper (instead of Escaper) makes Escapers.compose() easier.
    private static final Escaper NULL_ESCAPER =
            new CharEscaper() {
                @Override
                public String escape(String string) {
                    return Preconditions.checkNotNull(string);
                }

                @Override
                protected char[] escape(char c) {
                    return null;
                }
            };

    /**
     * Returns a builder for creating simple, fast escapers. A builder instance can be reused and each
     * escaper that is created will be a snapshot of the current builder state. Builders are not
     * thread safe.
     * <p>
     * <p>The initial state of the builder is such that:
     * <p>
     * <ul>
     * <li>There are no replacement mappings
     * <li>{@code safeMin == Character.MIN_VALUE}
     * <li>{@code safeMax == Character.MAX_VALUE}
     * <li>{@code unsafeReplacement == null}
     * </ul>
     * <p>
     * <p>For performance reasons escapers created by this builder are not Unicode aware and will not
     * validate the well-formedness of their input.
     */
    public static Builder builder() {
        return new Builder();
    }

    /**
     * A builder for simple, fast escapers.
     * <p>
     * <p>Typically an escaper needs to deal with the escaping of high valued characters or code
     * points. In these cases it is necessary to extend either {@link ArrayBasedCharEscaper} or {@link
     * ArrayBasedUnicodeEscaper} to provide the desired behavior. However this builder is suitable for
     * creating escapers that replace a relative small set of characters.
     */
    public static final class Builder {
        private final Map<Character, String> replacementMap = new HashMap<Character, String>();
        private char safeMin = Character.MIN_VALUE;
        private char safeMax = Character.MAX_VALUE;
        private String unsafeReplacement = null;

        /**
         * The constructor is exposed via the builder() method above.
         */
        private Builder() {
        }

        /**
         * Sets the safe range of characters for the escaper. Characters in this range that have no
         * explicit replacement are considered 'safe' and remain unescaped in the output. If {@code
         * safeMax < safeMin} then the safe range is empty.
         *
         * @param safeMin the lowest 'safe' character
         * @param safeMax the highest 'safe' character
         * @return the builder instance
         */
        public Builder setSafeRange(char safeMin, char safeMax) {
            this.safeMin = safeMin;
            this.safeMax = safeMax;
            return this;
        }

        /**
         * Sets the replacement string for any characters outside the 'safe' range that have no explicit
         * replacement. If {@code unsafeReplacement} is {@code null} then no replacement will occur, if
         * it is {@code ""} then the unsafe characters are removed from the output.
         *
         * @param unsafeReplacement the string to replace unsafe characters
         * @return the builder instance
         */
        public Builder setUnsafeReplacement(@Nullable String unsafeReplacement) {
            this.unsafeReplacement = unsafeReplacement;
            return this;
        }

        /**
         * Adds a replacement string for the given input character. The specified character will be
         * replaced by the given string whenever it occurs in the input, irrespective of whether it lies
         * inside or outside the 'safe' range.
         *
         * @param c           the character to be replaced
         * @param replacement the string to replace the given character
         * @return the builder instance
         * @throws NullPointerException if {@code replacement} is null
         */
        public Builder addEscape(char c, String replacement) {
            Preconditions.checkNotNull(replacement);
            // This can replace an existing character (the builder is re-usable).
            replacementMap.put(c, replacement);
            return this;
        }

        /**
         * Returns a new escaper based on the current state of the builder.
         */
        public Escaper build() {
            return new ArrayBasedCharEscaper(replacementMap, safeMin, safeMax) {
                private final char[] replacementChars =
                        unsafeReplacement != null ? unsafeReplacement.toCharArray() : null;

                @Override
                protected char[] escapeUnsafe(char c) {
                    return replacementChars;
                }
            };
        }
    }

    /**
     * Returns a {@link UnicodeEscaper} equivalent to the given escaper instance. If the escaper is
     * already a UnicodeEscaper then it is simply returned, otherwise it is wrapped in a
     * UnicodeEscaper.
     * <p>
     * <p>When a {@link CharEscaper} escaper is wrapped by this method it acquires extra behavior with
     * respect to the well-formedness of Unicode character sequences and will throw {@link
     * IllegalArgumentException} when given bad input.
     *
     * @param escaper the instance to be wrapped
     * @return a UnicodeEscaper with the same behavior as the given instance
     * @throws NullPointerException     if escaper is null
     * @throws IllegalArgumentException if escaper is not a UnicodeEscaper or a CharEscaper
     */
    static UnicodeEscaper asUnicodeEscaper(Escaper escaper) {
        Preconditions.checkNotNull(escaper);
        if (escaper instanceof UnicodeEscaper) {
            return (UnicodeEscaper) escaper;
        } else if (escaper instanceof CharEscaper) {
            return wrap((CharEscaper) escaper);
        }
        // In practice this shouldn't happen because it would be very odd not to
        // extend either CharEscaper or UnicodeEscaper for non trivial cases.
        throw new IllegalArgumentException(
                "Cannot create a UnicodeEscaper from: " + escaper.getClass().getName());
    }

    /**
     * Returns a string that would replace the given character in the specified escaper, or {@code
     * null} if no replacement should be made. This method is intended for use in tests through the
     * {@code EscaperAsserts} class; production users of {@link CharEscaper} should limit themselves
     * to its public interface.
     *
     * @param c the character to escape if necessary
     * @return the replacement string, or {@code null} if no escaping was needed
     */
    public static String computeReplacement(CharEscaper escaper, char c) {
        return stringOrNull(escaper.escape(c));
    }

    /**
     * Returns a string that would replace the given character in the specified escaper, or {@code
     * null} if no replacement should be made. This method is intended for use in tests through the
     * {@code EscaperAsserts} class; production users of {@link UnicodeEscaper} should limit
     * themselves to its public interface.
     *
     * @param cp the Unicode code point to escape if necessary
     * @return the replacement string, or {@code null} if no escaping was needed
     */
    public static String computeReplacement(UnicodeEscaper escaper, int cp) {
        return stringOrNull(escaper.escape(cp));
    }

    private static String stringOrNull(char[] in) {
        return (in == null) ? null : new String(in);
    }

    /**
     * Private helper to wrap a CharEscaper as a UnicodeEscaper.
     */
    private static UnicodeEscaper wrap(final CharEscaper escaper) {
        return new UnicodeEscaper() {
            @Override
            protected char[] escape(int cp) {
                // If a code point maps to a single character, just escape that.
                if (cp < Character.MIN_SUPPLEMENTARY_CODE_POINT) {
                    return escaper.escape((char) cp);
                }
                // Convert the code point to a surrogate pair and escape them both.
                // Note: This code path is horribly slow and typically allocates 4 new
                // char[] each time it is invoked. However this avoids any
                // synchronization issues and makes the escaper thread safe.
                char[] surrogateChars = new char[2];
                Character.toChars(cp, surrogateChars, 0);
                char[] hiChars = escaper.escape(surrogateChars[0]);
                char[] loChars = escaper.escape(surrogateChars[1]);

                // If either hiChars or lowChars are non-null, the CharEscaper is trying
                // to escape the characters of a surrogate pair separately. This is
                // uncommon and applies only to escapers that assume UCS-2 rather than
                // UTF-16. See: http://en.wikipedia.org/wiki/UTF-16/UCS-2
                if (hiChars == null && loChars == null) {
                    // We expect this to be the common code path for most escapers.
                    return null;
                }
                // Combine the characters and/or escaped sequences into a single array.
                int hiCount = hiChars != null ? hiChars.length : 1;
                int loCount = loChars != null ? loChars.length : 1;
                char[] output = new char[hiCount + loCount];
                if (hiChars != null) {
                    for (int n = 0; n < hiChars.length; ++n) {
                        output[n] = hiChars[n];
                    }
                } else {
                    output[0] = surrogateChars[0];
                }
                if (loChars != null) {
                    for (int n = 0; n < loChars.length; ++n) {
                        output[hiCount + n] = loChars[n];
                    }
                } else {
                    output[hiCount] = surrogateChars[1];
                }
                return output;
            }
        };
    }
}
